Water-in-oil emulsion explosives are well-known in the art. They are fluid when formed (and can be designed to remain fluid at temperatures at use) and are used in both packaged and bulk forms. They commonly are mixed with ammonium nitrate prills and or ANFO to form a "heavy ANFO" product, having higher energy and depending on the ratios of components, better water resistance than ANFO. Such emulsions normally are reduced in density by the addition of gas or air voids in the form of hollow microspheres or gas bubbles, which materially sensitize the emulsion to detonation. A uniform, stable dispersion of the microspheres or gas bubbles is important to the detonation properties of the emulsion. Gas bubbles, if present, normally are produced by the reaction of chemical gassing agents.
Chemically gassed water-in-oil emulsion explosives are well-known in the art. See, for example, U.S. Pat. Nos. 4,141,767; 4,216,040; 4,426,238; 4,756,777; 4,790,890 and 4,790,891. Chemical gassing agents normally are soluble in the inorganic oxidizer salt or discontinuous phase of the emulsion and react chemically in the oxidizer salt phase under proper pH conditions to produce a fine dispersion of gas bubbles throughout the emulsion. The timing of the addition of the gassing agent is important. The gassing agent or portion thereof that decomposes or reacts chemically in the oxidizer salt solution generally cannot be added to the oxidizer salt solution prior to formation of the emulsion or gassing would occur prematurely. Similarly, if an emulsion is to be subjected to further handling procedures, such as pumping into a borehole or mixing with ammonium nitrate prills or ANFO, then the chemical gassing reaction should not occur fully until after such handling occurs in order to minimize coalescence and/or escape of the gas bubbles. Further, after final placement of the explosive into a borehole, package or other receptacle, gassing should progress to completion in a desired time frame for the specific application or subsequent activities such as cooling, packaging or borehole stemming could interfere with the desired density reduction. Thus the gassing timing and rate must be optimized for a given application.
Since the gassing agent generally is added after the emulsion has been formed, the gassing agent must find its way into or otherwise combine with the discontinuous phase (oxidizer salt phase) of the emulsion in order to decompose or react chemically to produce gas bubbles. Thus it is important that the gassing agent be dispersed quickly and homogeneously throughout the emulsion. The ease by which the gassing agent finds its way into the oxidizer salt phase depends on the stability of the emulsion and on the type of emulsifier used. With a more stable emulsion and/or with particular types of emulsifiers, it is more difficult and thus takes longer or requires more mixing or shearing action to mix uniformly and combine the gassing agent solution with the oxidizer phase of the emulsion and thereby obtain gassing at a sufficiently high rate. This particularly is the case when polymeric emulsifiers are used, such as polyalkenyl succinic acid esters and amides, polyalkenyl phenolic derivatives and the like. These types of emulsifiers tend to form highly stable emulsions. Polymeric emulsifiers of this type are described in U.S. Pat. Nos. 4,357,184; 4,708,753; 4,784,706; 4,710,248; 4,820,361; 4,822,433; and 4,840,687. As used herein the term "polymeric emulsifier" shall mean any emulsifier wherein the lipophillic portion of the molecule is composed of a polymer derived from the linking of two or more monomers.
It has been found in the present invention that the addition of a surfactant that is soluble in the oxidizer salt phase, concurrently with the addition of the chemical gassing agent, significantly increases the rate of gas generation from the chemical gassing agent. The surfactant can be conveniently dissolved in the gassing agent solution. It can also be added as a separate solution or combined with another aqueous miscible more fully below, it is believed that the surfactant enables the gassing agent to enter the discontinuous phase more quickly, easily and uniformly, which thus allows the chemical gassing reaction to proceed at a faster rate.